One type of offshore drilling and production platform, generally called a TLP, utilizes tendons to support the platform. The tendons have lower terminations that connect to pilings on the sea floor. The upper ends connect to top connectors on the platform. The platform is de-ballasted after connection to the top connector, placing the tendons in tension.
One type of tendon consists of steel tubular joints of pipe connected together with welds or mechanical connections. The pipe has hollow interiors that are sealed from sea water to provide buoyancy. Bulkheads may be located within the interior, dividing the hollow interior in separate compartments sealed from each other. U.S. Pat. No. 6,851,894 discloses tubular sections having three different wall thicknesses. The upper section has a greater diameter but lesser wall thickness than an intermediate section, and the intermediate section has a greater diameter but lesser wall thickness than the lower section. Sealed bulkheads are not disclosed in this patent.
Another type of tether or tendon is a solid cable, preferably formed of composite fibers, such as carbon fibers. Typically, a composite tendon has an elastomeric jacket that encloses several bundles of fibers. A spacer or filler fills the interior space surrounding the fibers. Steel terminations are located on the ends of the separate rods or sections of a composite tendon for connecting the sections to each other.
Composite fiber tendons are generally smaller in diameter than steel tubular tendons and weigh less. However, they are less buoyant, such as being around 0.85 where 1.00 is considered neutral. Having solid interiors, composite fiber tendons are able to withstand high hydrostatic pressures. However, the lack of buoyancy limits the usefulness of composite fiber tendons in very deep water because a larger and more buoyant hull for the TLP is required. Also, fatigue of the upper portion of a composite fiber tendon can be a concern because of the high bending moments caused by TLP lateral motion.
As TLP platforms are located in deeper waters, providing steel tubular tendons that can resist the hydrostatic pressure becomes an increasingly difficult problem. Composite fiber tendons have an advantage of being able to resist very high hydrostatic pressure, but are heavy in water due to the lack of buoyancy.